1. Field of the Invention
The invention relates to formwork.
2. Background Information
Formwork is used for casting concrete structures in situ or modular concrete building blocks and panels. Traditionally, especially for structures, the formwork is made up of suitable pieces of plywood board and held together to provide an open-topped cavity into which concrete is poured and allowed to cure. The panels may be removed before the concrete is fully cured, typically 20 to 30 hours after the pouring. Metal formwork panels may also be used and have the advantage, generally stated, of being re-usable without modification but the disadvantage of being more expensive and much heavier and so less easy to manhandle. Metal formwork is less adaptable to form complex shapes or formwork “tailor-made” for each structure requiring to be cast in the progressive construction of a building, for example.
For both wooden and metallic formwork it is a normal requirement to create a good, even and consistent surface finish in the cast panels or blocks. This is especially the case where the surface is designed to be later clad with tiles. Imperfections and structural weakness in the surface lead to egress of dampness behind the cladding and ultimate failure of the tile bonding. It has been found that wood is often preferable to create a good surface finish because wood absorbs free surface water and bleeding water released from the poured concrete.
Typically, a release agent must be applied to the wood or metal formwork so that it can be separated from the cured concrete structure without damage to the surface finish. Residue from the release agent is left on the finished surface. Most release agents are manufactured from organic compounds, which are a food source for biological organisms such as fungi, bacteria, algae and lichens This can lead to accelerated growth of these organisms on the concrete surface.
A number of chemicals are added to concrete to control setting time, plasticity, pumpability, water content, free-thaw resistance, strength and color, etc. It is also known to add fungicides, germicides and insecticides to the concrete to help control for biological organisms.
However, these chemical admixtures can result in the off-gassing of small quantities of formaldehydes and other chemicals from the finished concrete. For chemically sensitive clients it may be a requirement to minimize the use of these admixtures.
It is known to use porous plastic sheeting to cover the exposed surfaces of a formwork. This is known in the art as Controlled Permeability Formwork or CPF. Water can leave the poured cavity surfaces through the plastic sheeting. Additionally, no release agent is required and thus the problem of biological organisms is minimized. The disadvantage with the CPF system is that the porous plastic sheeting is expensive and a drainage system is required to channel away water that passes; through the sheeting.